This invention relates to preventing corrosion of susceptible substrates as ferrous metals through electrodeposition of protective coatings and includes, in particular, electrodepositable primers that retard corrosion of such substrates, even when formulated to be free of such inhibiting pigments as chromates, while at the same time providing smooth, desirable coatings after baking.
The corrosion of steel immersed in an aqueous environment can often be slowed to an acceptable rate by the introduction of corrosion inhibitors soluble in the aqueous medium. In the case of steel structures subjected intermittently to a corrosive environment, or under conditions which preclude effective incorporation of a corrosion inhibitor into the corrosive medium, protective organic coatings are often used. One of the major functions of these coatings is to act as a reservoir for sparingly soluble inorganic corrosion inhibitors which act to slow the rate of corrosion at coating defects and other incipient anodic corrosion sites. Among the most effective inhibitors are inorganic chromates whose safety has been questioned by some. While the elimination of these pigments may therefore be desirable, it is found that in the absence of effective corrosion inhibiting pigments extensive adhesion failure adjacent to coating defects and flaws can take place, resulting ultimately in the uncontrolled spread of corrosion. The principal cause of corrosion-induced adhesion failure is generally thought to be displacement and/or degradation of the coating resin by cathodically produced hydroxide ion. Corrosion inhibitors suppress this effect by slowing the overall rate of corrosion, limiting the amount of alkali produced. In the absence of corrosion inhibitors it is therefore necessary that the coating resin be exceptionally resistant to displacement and/or degradation by corrosion-produced hydroxide.
A principal objective is the provision of resins suitable for electrodeposition processes and additionally highly resistant to alkali degradation. A further objective is the provision of organic coating formulations which enable ferrous substrates to be protected from corrosion without the incorporation of possibly deleterious corrosion inhibitive pigments. It is further desired that the organic coatings be suitable for use as primers on automotive steel substrates and that the application and handling characteristics be compatible with conventional electrodeposition commercial paint operations and practices. It is still further desired that the coatings exhibit, after bake, a smooth, unrippled appearance.
Commonly assigned U.S. Ser. No. 938,677, concurrently filed herewith in the name of Dickie and Holubka disclose crosslinking compositions that, upon application, and bake, are found to desirably retard corrosion of ferrous substrates and also, when at least partially neutralized, may be electrodeposited. Such electrodepositable crosslinking compositions, however, are not wholly satisfactory insofar as they leave an uneven coating surface after deposition and bake within conventional schedules. Moreover, while inclusion of certain flow control agents as poly- (butylacrylates) lead to more smooth coatings, the coatings remain tacky after bake. In this invention, coatings with both uniformly smooth and non-tacky surfaces are achieved after baking while desirable corrosion protection properties as well as aqueous bath stability are maintained.